Hollow fiber membrane modules can provide a large membrane area per unit volume. The hollow fiber membrane modules are thus applied in many fields of fluid treatment, for example, demineralization of brackish water and sea water using a reverse osmosis membrane, primary purification treatment of ultrapure water, removal of low molecular-weight organisms such as agricultural chemicals and polysaccharides using a nanofilter, concentration and demineralization of enzymes using a ultrafiltration membrane, manufacture of water for injection, recovery of electroplating coats, final filtration of ultrapure water, waste water treatment, clarification of river water, lake water, and river-bed water, purification, sterilization, and clarification of chemicals using a microfiltration membrane, and oxygen separation, nitrogen separation, hydrogen separation, and carbon dioxide gas separation using a gas separation membrane.
In recent years, in allowing river water, river-bed water, and the like to be used as purified water, hollow fiber membranes and hollow fiber membrane modules have been more widely used as a clarification method instead of coagulating sedimentation and sand filtration. Thus, new, high-performance hollow fiber membranes and hollow fiber membrane modules have been proposed.
Most conventional external pressure-type hollow fiber membrane modules pressurizing raw water via a hollow fiber membrane to obtain permeated water are constructed as follows. Hollow portions are sealed to an adhesive fixing portion positioned at the bottom of the module and partitioning hollow fiber membranes from a module housing in a liquid-tight manner. A plurality of raw water introduction holes are formed in the adhesive fixing portion so that raw water can be fed parallel to the hollow fiber membranes through the holes. Permeated water is sampled through the opening of each of the hollow fiber membranes in the end surface thereof located at the top of the module. Concentrated water containing suspended substances is discharged through a concentrated water discharge nozzle on a side surface of the upper portion of the module (See PATENT DOCUMENTS 1 and 2).
An example of the structure of such an external pressure-type hollow fiber membrane module is shown in FIG. 17. In FIG. 17, a large number (in this case, for simplification, three) hollow fiber membranes 105 are housed in a module case 104. At the top of the case, the hollow fiber membranes and the module case are adhesively fixed together in a liquid-tight manner by an adhesive fixing portion 106. The terminals of the hollow fiber membranes are open so as to enable a liquid to pass through. Permeated water is collected in a cap 101 and pumped upward through a permeated water sampling port 112 for sampling.
On the other hand, at the bottom of the case, the hollow fiber membranes are adhesively fixed to the module case in a liquid-tight manner by an adhesive fixing portion 107. The terminals of the hollow fiber membranes are closed. A plurality of raw-water introduction holes 108 are open in the lower adhesive fixing portion 107 so that any one of raw water, compressed air, and a mixed flow of raw water and compressed air can be fed through the holes 108. Here, a liquid flow during normal filtration will be described. Raw water flows from a raw-water supply port 110 formed in a lower cap 103, via the raw-water introduction holes 108 into the module case. Most of the raw water permeates the hollow fiber membranes 105. The resulting permeated water flows through the upper openings of the hollow fiber membranes via the cap 101 and sampled through the permeated water sampling port 112. Furthermore, part of the raw water is concentrated, and the concentrated water is discharged through a concentrated water discharge nozzle 111 on the side surface of the upper portion. At this time, depending on the quality of the raw water, a method can be adopted which discharges the concentrated water only during physical washing such as flushing, back washing, or aeration flushing rather than continuously discharging the concentrated water.
When miniaturized in order to evaluate the performance of the membranes or check for filtration stability, a hollow fiber membrane module constructed as described above may have an effective membrane length of about 1 m.
Furthermore, if the hollow fiber membrane module is used for an actual large-scale water clarification treatment, the effective length of the hollow fiber membranes is normally set to a larger value of about 2 m in order to reduce the installation area of an arrangement rack in which the membrane module is arranged or to increase the area of the hollow fiber membranes per unit volume. However, since the conventional hollow fiber membranes exhibit low permeation performance, a pressure drop in the hollow portion on the permeated water side is small. The conventional module is thus used without posing a problem in a practical sense.
However, recently, with an increase in the number of applications of membrane filtration modules for purified water, the permeation performance of the hollow fiber membranes has been improved. On the other hand, what is called one-sided water collection modules have frequently failed to offer a level of permeation performance otherwise exhibited by the hollow fiber membranes; in the one-sided water collection module, permeated water is sampled only through the openings of the hollow fiber membranes located at the top of the module.
Thus, a structure is known which has a module structure with a communication portion through which permeated water flows from one side to the other side in order to improve the effective utilization factor of the hollow fiber membranes; in this structure, permeated water can be sampled through the opposite ends of the hollow fiber membrane module (see, for example, PATENT DOCUMENTS 3 to 6). FIGS. 4, 5, and 9 in PATENT DOCUMENT 3 show that compressed air is introduced through a lower air introduction hole 19 to vibrate the hollow fiber membranes.
Such an introduction structure enables a fluid with a relatively low viscosity such as compressed air to be evenly distributed with a relatively low pressure drop. However, a viscous fluid such as water containing suspended substances results in a very heavy drop in the pressure required for a necessary supply. The mere feeding of the fluid into the hollow fiber membrane module requires at least 100 KPa. This supply pressure is equivalent to or higher than that required for the normal supply of raw water, that is, 50 to 100 KPa. Thus, it is unpractical to apply the structure with the introduction holes for air only to water without change.
Moreover, in a method for manufacturing the module structure, as described in PATENT DOCUMENT 4, a plurality of holes are formed along the outer circumference of the module case, and a partitioning plate with holes formed therein and a hollow fiber membrane bundle are housed in the module case at a time. Moreover, the holes in the partitioning plate are placed opposite the corresponding holes in the module case. The partitioning plate and the module case are adhesively fixed together using bolts with flexible tubes. Moreover, the bolts with the flexible tubes are removed. The method thus requires a very complicated assembly operation. It is thus difficult to apply the method to, for example, a membrane filtration process for river water which allows permeated water to be inexpensively obtained.
PATENT DOCUMENT 5 discloses a structure similar to those in PATENT DOCUMENTS 3 and 4. However, in this case, unless as seen in a sectional view, the air introduction holes are formed symmetrically with respect to a center axis and at equal intervals, the supply of air or water is not uniform. The symmetric arrangement at the equal intervals requires a complicated assembly operation as is the case with PATENT DOCUMENT 4.
Furthermore, PATENT DOCUMENT 6 discloses a structure in which a large number of raw water introduction holes extend from a site corresponding to a supply source pipe portion for raw water drilled from the substantial center of the outer end surface of one adhesive fixing portion, directly to the other adhesive fixing portion so as to allow raw water to be fed perpendicularly to the hollow fiber membranes. In this case, the raw water is fed perpendicularly outward from the center of the hollow fiber membrane bundle. Thus, disadvantageously, suspended substances are accumulated between the central hollow fiber membranes, preventing the raw water from being fed to the outer circumference of the bundle.    PATENT DOCUMENT 1: JP-A-07-171354    PATENT DOCUMENT 2: JP-A-09-220446    PATENT DOCUMENT 3: JP-U-63-111901    PATENT DOCUMENT 4: JP-A-64-090005    PATENT DOCUMENT 5: JP-U-03-119424    PATENT DOCUMENT 6: JP-A-53-035860